Manufacturing automation has become increasingly important as the need to remain high throughput increases. In manufacturing of a product, many processes may be needed. For example, semiconductor manufacturing may include over 400 processing steps involving more than 100 different tools. Furthermore, the process route can include a high level of reentrance in which the same tool or tool types are used. An important aspect in manufacturing automation is material handling. To facilitate material handling, automated transport systems are employed.
Conventional automated transport systems are used in, for example, a semiconductor manufacturing plant typically comprises a plurality of bays. Each bay includes a plurality of tools which are used to process wafers. Transport vehicles, such as overhead transport (OHT) are provided for automatic transfer of wafers contained in a carrier. A stocker which temporarily stores carriers is provided for each bay. A transport system controller controls the movement of carriers within a bay or between bays.
FIG. 1 shows the material flow of a conventional manufacturing facility having an automated transport system. At step 105, a manufacturing execution system (MES) polls the tools sequentially within a bay. The polling is achieved with a “watchdog”. The MES can have multiple watchdogs, which each monitoring a respective group of tools. For example, each watchdog will monitor a certain number of tools, such as 10. The watchdog will go and check the status of the 10 tools to see whether a tool requires an OHT for carrier delivery or pickup. When a tool has a status which indicates that it requires the services of the automated transport system, it can be either to process another lot (A) or to pick up a processed lot (B). In the case where another lot is to be processed, the MES instructs the automated transport system to dispatch a vehicle to the stocker to retrieve a lot at step 115. The vehicle delivers the retrieved lot to the tool for processing at step 125. A situation may occur where the lot to be processed is located in a stocker of another bay. For such situation, the controller dispatches a vehicle in the main bay to retrieve the lot at step 155 and delivers it to the appropriate bay at step 165. Once the lot is in the appropriate bay, the process proceeds as in A. On the other hand, when a processed lot is to be picked up, the controller dispatches a vehicle to the tool. The vehicle retrieves the processed lot from the tool at step 135 and delivers it to the stocker at step 145. According to conventional automated transport systems, at least one stocker is necessary to serve as an intermediary.
The typical cycle time (e.g., time period from the process completion of a first lot to the arrival of a second lot for processing) of conventional automation system is greater than 11 minutes. It is desirable to increase manufacturing productivity of automation system by improving cycle time.